1. Field of the Invention
The invention relates to aqueous ink compositions which are stably ejected in a jet for an extended period of time, a process for preparing the same, and an ink jet recording method using the same. More particularly, it relates to an aqueous ink composition which is stable in ejecting through ink jet nozzles in an ink jet recording and to a process for preparing ink having improved ejecting properties.
2. Related Art
An ink jet recording unit is mounted on a computer terminal, a printing calculator, a facsimile, or similar device. The printing performance of the ink jet recording unit is largely governed by the nature and quality of the ink. For example, if an ink component coagulates as a result of water evaporation from the ink, or if a material forming the ink path through the ink jet head dissolves and precipitates in ink, the ink jet nozzles can become clogged and fail to eject ink droplets.
JP-A-9-123437 teaches adding urea to ink compositions to improve water retention. According to the technique disclosed, evaporation of the water content is suppressed to prevent the ink component from coagulation. Also, the solubility of the material constituting the ink path through the ink jet head is increased to prevent the material from precipitating. Although addition of urea to ink compositions is effective in preventing clogging of the ink eject, it has been learned that urea can decompose to detrimentally alter the ink composition or its physical properties. This can result in deteriorated printing quality when a urea-containing ink composition is left to stand at high temperatures or for extended periods at moderate temperatures. It has hence been proposed to add not only urea but ammonium ions to ink compositions thereby suppressing urea decomposition as disclosed in JP-A-5-17714.
However, where urea is added to an ink composition containing an azo dye as a colorant, reduction in printing performance can result for another reason. That is, there is a tendency for the ink composition to be supersaturated with nitrogen in the ink passageway. Thus, the composition is prone to forming bubbles when subjected to shocks, such as a pressure shock attendant upon ejecting. In such cases, the energy for ejecting ink is partially absorbed by the gas bubbles, resulting in a failure to eject ink stably.
In one aspect, the present invention provides aqueous ink compositions containing an azo dye and urea which maintains high printing performance while preventing bubble formation in the course of use.
In another aspect, the invention provides a process for preparing such aqueous ink compositions.
In a further aspect, the invention provides an ink jet recording method using such aqueous ink compositions.
In some preferred embodiments, the invention provides an aqueous ink composition comprising urea, a colorant comprising an azo dye, and water. Such a preferred composition has a dissolved ammonium ion concentration of not more than 200 ppm, and a dissolved nitrogen content of not more than 5 ppm.
The term "urea" as used herein includes not only urea itself, but analogous compounds such as thiourea. The term "aqueous ink" as used herein denotes not only inks containing water as the sole solvent, but also those containing mixed solvents such as water and a water-soluble organic solvent. The language "comprising urea, a colorant comprising an azo dye, and water" implies that the ink can contain other components, such as viscosity modifies and rust inhibitors, if desired.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable materials and methods are described below. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods and examples are illustrative only and not intended to be limiting.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.